1. Field of the Invention
This invention relates to an image forming apparatus such as a copier, a printer or a compound machine having also the function of these, and particularly to the sheet conveyance control by registration rollers disposed upstream of image forming means for forming an image on a sheet.
2. Description of the Prior Art
In an image forming apparatus such as a copier, the productivity and the quality of image of the apparatus are very important specifications, and in recent years, higher productivity and a higher quality of image have been required. The first copy time (hereinafter referred to as the Fcot) from the start of image formation on the first sheet till the end of the image formation is also a very important specification, and the shortening of the Fcot is required of the apparatus.
To increase the productivity and shorten the Fcot, it is usual to increase the image forming speed of the image forming apparatus. However, when the image forming speed is increased, the driving load, the rotating speed, etc. of each portion of the apparatus rise and therefore, a countermeasure such as increasing the rigidity of each portion of the apparatus becomes necessary and as the result, there arises the problem that the apparatus becomes bulky or the costs of parts and the production cost rise.
Also, regarding digital copiers and printers which have been rapidly spreading in recent years, when the image forming speed is increased, the number of revolutions of a polygon mirror which is the writing portion of a laser must be made very high. Therefore there may arise the problem that the cost of the polygon mirror becomes very high or the image is deteriorated because the rotation of a motor is not stable. There is also used the technique of restraining the number of revolutions of the polygon motor and using a plurality of writing lasers, but again this technique gives rise to the complication of the apparatus and the complication of and an increase in electrical parts. Further, in this case, in order to realize a higher quality of image, there arises the necessity of the high accuracy of many parts typified by the parts of an optical system, and this results in a great rise of production cost.
Therefore, in order to obtain the maximum productivity of the apparatus for the image forming speed, there has heretofore been used the technique of reducing the conveyance interval of sheets during image formation (during image transfer) as far as possible.
However, with regard to registration rollers provided upstream of image forming means, the time for ramming a sheet against the stopped registration rollers to thereby form a loop becomes necessary in order to effect the correction of the skew feeding of the sheet, and it is necessary to widen the conveyance interval of sheets by this loop forming time.
There is also seen an apparatus in which the control of changing the speed for conveying sheets is effected and the conveying speed is changed so as to convey the sheets at a speed higher than the image forming speed in the sheet conveying path from a sheet containing portion such as a sheet cassette to registration rollers to thereby realize the narrowing of the sheet conveyance interval. In such an apparatus, there is often seen a case where conveying rollers are driven by a stepping motor, but the vibration of the stepping motor for driving may cause the vibration of a laser unit or an optical unit and further, even image forming means and as the result, the deterioration of copy images may arise as another problem. Further, when the conveying speed is to be changed over, the pulse of the stepping motor is changed, but again at this time, the vibration of the stepping motor occurs and conceivably, the vibration of the sheet and the vibration of a conveyance driving unit affect image formation.
So, the present invention has as its object to provide a sheet conveying apparatus designed such that the control of changing the speed for conveying a sheet is effected by registration rollers and that the vibration of the sheet and the vibration of a motor for driving conveying rollers are not transmitted to image forming means and transferring means, and an image forming apparatus having the same.
The present invention is an image forming apparatus comprising:
image forming means for forming an image on a sheet;
separating and feeding means for feeding sheets stacked on sheet supporting means one by one toward the image forming means;
sheet conveying means for conveying the sheets fed by the separating and feeding means;
registration rollers of a variable rotating speed disposed upstream of the image forming means with respect to a sheet conveying direction; and
sheet conveyance controlling means for causing the registration rollers to effect the conveyance of the sheets at a speed higher than the image forming speed in the image forming means, and thereafter decelerating the registration rollers so as to assume substantially the same speed as the image forming speed at timing earlier than latent image forming timing in the image forming means.